This is an interesting question because to answer it we have to look at the climate on a global scale. Simply put, rainfall isn’t going to decrease or increase the same all over the planet as it warms. Some places will get more rain, some places will get less, and some will stay the same as they are now. In fact, places where lots of evaporation occurs (like the sub-tropics), are likely to get more rainfall as a result of global atmospheric warming, since a warmer atmosphere can hold more water vapor. However, places like the Sahara desert, where evaporation is scarce and temperatures are already searing hot, is likely to get less! So, wet places get wetter, dry places get dryer, and the frequency of extreme weather events (huge rainstorms or huge droughts, like we are having in Southern California) is likely to increase as the climate rapidly warms.